The present invention relates to switching circuits in general, and in particular to a switch mode converter and a primary switch drive therefor.
The term "switch mode converter" is used herein in a broad sense, and encompasses DC to DC, AC to DC, AC to AC and DC to DC switch mode converters. The term "switch" is also used herein in a broad sense and can encompass two or more individual switch elements.
DC to DC switch mode converters convert a DC input voltage to a regulated DC output voltage by chopping the DC input voltage with a primary switch means to convert it to an AC voltage and then rectify and filter the AC voltage to provide the DC output voltage. A transformer or some other means may be provided to change the level of the chopped AC voltage so that the DC output voltage is obtained at the desired level after rectification and filtering. The DC output voltage is regulated by applying control signals to the primary switch means, which includes one or more primary switches, to vary its duty cycle and hence the duty cycle of the AC voltage obtained from chopping.
It is highly desireable that the primary switches be turned on and off in as short a time as possible and as efficiently as possible to protect the primary switches, improve converter performance and reduce the power dissipation of the converter.
Many switch mode converters utilize transistors as the primary switches. Due to storage time characteristics, transistors exhibit a delay between receiving a signal to turn the transistor off and actually turning off. This delay which is dependent on the particular transistor and the current it is conducting, can impair converter performance and efficiency as well as subject the transistor and other parts of the converter to damage.
In a known transistor switching circuit, a transistor is coupled as a primary switch and switched on and off by a controller means including a transformer having a primary winding and a secondary winding, switch means for opening and closing a current path through the transformer primary winding and a current supplying means coupled to the transformer primary winding. The current supplying means comprises a resistor coupled between a voltage source and the primary winding, and a capacitor coupled between ground and the connection of the resistor to the primary winding so as to charge through the resistor and discharge through the primary winding. The transformer secondary includes a main portion coupled across the base/emitter of the transistor to turn it on and off in response to current flow in the primary winding and a feedback portion coupled to the emitter of the transistor to induce current in the main secondary winding portion after the transistor has been turned on to maintain it turned on.
When the switch means closes the current path, current builds up in the primary winding as produced by the voltage across the capacitor and the inductance of the primary winding and causes energy to be stored in the core of the transformer. When the switch means opens the current path, the energy stored in the core of transformer induces current in the main secondary winding portion which flows into the base of transistor to turn it on. The feedback winding portion then provides positive feedback to the main winding portion to maintain the transistor on with a base current essentially proportional to its collector current. During the time that the switch means opens the current path through the transformer primary winding, the capacitor is recharged from the voltage source. When the switch means again closes the current path, the voltage across the charged capacitor generates a reverse polarity voltage across the primary winding which induces a reverse voltage across the base/emitter of the transistor and a negative drive current to the base of the transistor. In order to prevent saturation of the transformer, the current supplying means is made self-adjusting by operation of the circuit and the voltage across the capacitor supplies volt-seconds to the transformer sufficient to cause switching of the transistor and insufficient to saturate the transformer. The volt-seconds provided to the transformer primary winding are the result of a compromise between obtaining fast transistor switching and preventing transformer saturation. Therefore, the negative base current provided to the transistor induced by the charge on the capacitor builds up relatively slowly due to this compromise and can delay turn-off of the transistor. Simply continuously increasing the volt-seconds supplied to the transformer primary winding in order to induce more turn-off current out of the base of the transistor could result in transformer saturation, and simply increasing the size of the capacitor and/or the voltage source does not provide faster turn-off of the transistor at the switching speeds typically used in switch mode converters. Also, increasing the size of the capacitor or making other changes in the transformer primary circuit could affect the transformer secondary output signals so that overall performance can be degraded quite easily.
There is thus a need to switch semi-conductor switches on and off efficiently and more rapidly while permitting switching at high speeds and without detracting from other performance characteristics of the switching circuit.